ABSTRACT
Objective:To discuss the strategy of fighting and containing COVID-19. Method:The speed of the spreading of the ordinary strain in Dalian on Dec. 15， 2021 caused by carrying cold-chain goods from a Russia-registered freighter and the Delta strain caused by imported cases in Guangzhou during May 20 to Jul. 23， 2021 was compared. Results:The incubation period of the first generation of the imported Delta strain of COVID-19 was 4 days， which was 5 days shorter than that of the ordinary strain of COVID-19； The time from the exposure to the Delta strain to spreading to the next person was 1/3 days shorter than that of the ordinary strain of COVID-19. The inactivated vaccine needs at least 8 days for recall immune response meanwhile the mRNA vaccine needs at least 4 days， which could not completely block the COVID-19 virus， especially the Delta strain， which can attack with an ultra-short incubation period. Meanwhile， some COVID-19 strains have an incubation period longer than 14 days， which was another reason for the outbreak. Conclusion:The time from the exposure to the COVID-19 Delta strain to spreading to the next person is 1/3 day shorter that of the ordinary strain of COIVD-19. Measures of full vaccination， physical quarantine and wearing masks in public should be taken to fight the epidemic of COVID-19， especially the Delta strain （or other variants） with an ultra-short or an ultra-long incubation period which can escape quarantine.